Investigate the key features of stellar spectra and describe how these are used to classify stars
- Stars are divided into seven spectral classes based on their surface temperatures: O, B, A, F, G, K and M.
- This system of classification was developed by Annie Jump Cannon.
- O is the hottest class.
- M is the coolest class
| Spectral class | Surface temperature / K | Colour | Spectral Features |
|---|---|---|---|
| O | 28,000 – 50,000 | Blue | Ionised helium lines Strong UV component |
| B | 10,000-28,000 | Blue | Neutral helium lines |
| A | 7500-10,000 | Blue-white | Strong hydrogen lines Ionised metal lines |
| F | 6000 − 7500 | White | Strong metal lines Weak hydrogen lines |
| G | 5000 − 6000 | Yellow | Ionised calcium lines Metal lines present |
| K | 3500 − 5000 | Orange | Neutral metals dominate Strong molecular lines |
| M | 2500 − 3500 | Red | Molecular lines dominate Strong neutral metals |
*Metal refers to any element apart from Hydrogen and Helium
- Each class is divided into ten further subdivisions using the numbers from 0 to 9.
- Our sun is a G2 star.
- The hotter a star is, the shorter its dominant wavelength is.
- Thus, by observing wavelengths through spectroscopy, temperature can be determined and stars can be classified.
Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)